Hi Jenny, although i'm sure this isn't what you want to hear, I fear you will alienate your son if you don't accept his choice of partner. I appreciate that you might like his wife, and indeed there is no need to take sides, but if you decline to meet wit his new lady when he's obviously so keen to introduce her I think that's a slight he might not easily forgive.
Look at it this way, you and she are likely to be the two most important women in his world, and you dismissing his girlfriend as something which might not be around for long will insult that special feeling he has for her and lessen the special feelings he has for you.
I'd personally meet her, be nice, be warm and be polite, she might one day be your future daughter in law. x